Metal surface treatment aqueous solution and method for inhibiting whiskers on a metal surface

ABSTRACT

A tin or tin alloy plating film surface treatment aqueous solution that can reduce whiskers on the surface of a tin or tin alloy plating film, and can provide a favorable tin or tin alloy plating film using a simple method for tin or tin alloy plating films that are used on electronic components.

The present invention relates to a surface treatment solution formetals, and particularly tin or tin alloy plating films, and to a methodfor treating the surface of tin or tin alloy plating films. In furtherdetail, the present invention relates to a method of preventing theoccurrence of whiskers on a film surface when a tin or tin alloy platingfilm is formed by a tin or tin alloy plating method, and also relates toa treatment solution thereof.

Tin or tin alloy plating is widely used in electronic components such ascomponents which require electric connecting, such as chip components,quartz oscillators, connector pins, lead frames, printed circuit boards,and is also used as an etching resistant in the manufacturing process ofsemiconductor devices and printed boards because of the excellentconnecting properties, low cost, electrical properties, and solderingproperties.

Without treatment, a plating film obtained by tin or tin alloy platingwill over time form whisker-like metal deposits known as whiskers on thesurface. The occurrence of whiskers is known to be more prominent on tinor tin alloy plating films on a copper base. When whiskers occur on thesurface of the tin or tin alloy plating film formed on the substratesurface of an electronic component, or the like, there is a possibilitythat an electrical short could occur. Therefore, tin-lead alloy platingwhich utilizes the effect of lead in inhibiting the oxidation of tin andthe growth of whiskers has conventionally been used on components whereelectrical connections are necessary. However, recently, the toxicity oflead is seen as a problem, and the use of lead in electronic componentshas become restricted. Therefore, there is a need for lead free tin ortin alloy plating.

In order to inhibit the occurrence of whiskers, a high-temperaturetreatment, or in other words, a reflow process is performed afterforming the tin or tin alloy plating film. However, while the reflowprocess is recognized as having an effect of inhibiting the occurrenceof whiskers, intermetallic compounds will form between the tin and thebase metal because of the heat, and therefore there is a problem thatthe amount of pure tin in the tin film is reduced, and this may have alarge effect on the connection reliability after mounting. Therefore, atin plating film surface treatment method that can inhibit theoccurrence of whiskers without performing a heat treatment or withrelaxed heat treatment conditions is desirable.

Various types of solutions and surface treatment methods for treatingthe surface of tin films have been used in the past as methods fortreating the surface of tin plating films. For example, JP2007-56286discloses a tin plating film surface treatment aqueous solution fortreating the surface of a tin plating film, containing an ammonium saltof phosphoric acid, a salt of polyphosphoric acid, or a salt of maleicacid, or the like, and having a specific pH. This document does presenta salt of maleic acid, but at a pH below 4.5, desirable results cannotbe achieved, the claimed elements of the present invention are notsatisfied, and the useful effects of the present invention are notshown. JP2007-197791 discloses a post-plating treatment compositioncontaining a compound having an amino nitrogen with at least two or moremethylene groups bonded to a phosphonic acid group, or salt of theaforementioned compound as an essential ingredient. This document doesnot disclose the compound that is used with the present invention, northe use of a surface treatment solution as a surface treatment agent fortin plating. Furthermore, in tests performed by the present inventors,when a tin plated surface is treated with the treatment solutioncontaining the aforementioned compound, the plating film will corrodeand the base will become exposed, thus proving that the compound is notpreferable. JP2006-28610 discloses a method of forming a tin platingfilm including a peeling step of peeling a part of the tin plating film,a discoloration preventing treatment step, and a heat treatment step, inthat order. This document has a peeling step in order to preventdiscoloration during a heat treatment step by removing tin atoms whichhave adhered to regions other than the intended regions, and onlyelectrolytic peeling is shown as a peeling step, and thus differs fromthe constitution of the present invention. JP2004-300466 discloses aplating surface posttreatment solution wherein a carboxylic acidderivative of 2-mercaptobenzothiazole is dissolved in an organic solventsuch as alcohol and then added to water. However, the solubility of acarboxylic acid derivative of 2-mercaptobenzothiazole in water is poorand the joint use of an alcohol is required as shown in JP2004-300466,and thus the actual plating process is complicated. JP2006-307343discloses a method of bringing a composition comprising phosphoric acidor salt thereof into contact with a metal. This document does notdisclose the compound that is used with the present invention, nor theuse of a surface treatment solution as a surface treatment agent.

An objective of the present invention is to provide a tin or tin alloyplating film surface treatment aqueous solution that can reduce whiskerson the surface of a tin or tin alloy plating film, and can provide afavorable tin or tin alloy plating film using a simple method for tin ortin alloy plating films that are used on electronic components.

As a result of diligent research to achieve the aforementionedobjectives, the present inventors discovered that an aqueous solution towhich specific organic compounds containing a carboxyl group are addedhas select advantages compared to aqueous solutions containing othercompounds, and thus the present invention was achieved. In other words,the occurrence of whiskers on a tin or tin alloy plating film caneffectively be prevented by a simple method by treating with the surfacetreatment aqueous solution of the present invention.

The first invention of the present application provides a tin or tinalloy plating film surface treatment aqueous solution for treating thesurface of tin or tin alloy plating films, comprising an organiccompound with at least one carboxyl group in the molecule, and whereinthe pH is 2.5 or lower. The organic compound having at least onecarboxyl group and the molecule is preferably an organic compound whichhas two or more carboxyl groups in the molecule and which does not havea nitrogen atom in the molecule. The organic compound with two or morecarboxyl groups in the molecule and without a nitrogen atom in themolecule is preferably malic acid, maleic acid, or citric acid.Furthermore, the tin or tin alloy plating film surface treatment aqueoussolution of the first invention of the present application preferablyalso contains a nitrogen compound.

The second invention of the present application provides a tin or tinalloy plating film surface treatment method for treating the surface ofa tin or tin alloy plating film with the tin or tin alloy plating filmsurface treatment aqueous solution according to the first invention.

The third invention of the present application provides a method ofpreventing whiskers on the surface of a tin or tin alloy plating film bytreating the surface of a tin or tin alloy plating film with the tin ortin alloy plating film surface treatment aqueous solution according tothe first invention.

The fourth invention of the present application provides a method ofmanufacturing electronic components with a tin or tin alloy platingfilm, comprising a step of treating the surface of a tin or tin alloyplating film with any of the tin or tin alloy plating film surfacetreatment aqueous solutions according to the first invention, after astep of plating a substrate with tin or tin alloy.

The fifth invention of the present application provides an electroniccomponent comprising a tin or tin alloy plating film, manufactured by anelectronic component manufacturing method that includes a step oftreating the surface of a tin or tin alloy plating film with a tin ortin alloy plating film surface treatment aqueous solution according tothe first invention, after a step of plating a substrate with tin or tinalloy.

The abbreviations used throughout this specification have the followingmeaning unless otherwise designated. g=gram; mg=milligram; ° C.=degreeCelsius; m=meter; cm=centimeter; μm=micrometer, L=liter; mL=milliliter.The range of all values includes the boundary values, and may becombined in any order. The terms “plating solution” and “plating bath”used throughout this specification are used interchangeably and have thesame meaning.

The tin or tin alloy plating film surface treatment aqueous solution ofthe present invention contains water and an organic compound with atleast one carboxyl group in the molecule, and has a pH of 2 or lower.Examples of organic compounds which have at least one carboxyl group inthe molecule include citric acid, malic acid, maleic acid, oxalic acid,glutaric acid, tartaric acid, lactic acid, gluconic acid, picolinicacid, and iminodiacetic acid, and the like, and salts thereof are alsoacceptable.

Of the organic compounds having at least one carboxyl group in themolecule, those which have two or more carboxyl groups in the moleculeand which do not have a nitrogen atom in the molecule are preferable.Organic compounds which satisfy these conditions include, for example,citric acid, malic acid, maleic acid, and the like, and salts thereofare also acceptable. Malic acid and maleic acid are particularlypreferable.

These organic compounds which have at least one carboxylic acid in themolecule are present as a compound in the surface treatment aqueoussolution in a range between 0.1 and 50 g/L, preferably in a rangebetween 5 and 30 g/L, and more preferably in a range between 5 and 20g/L.

The tin or tin alloy film surface treatment aqueous solution of thepresent invention must have a pH of 2.5 or lower. If the pH is greaterthan 2.5, the occurrence of whiskers will increase even though anorganic compound having at least one carboxyl group in the molecule isused, and therefore this is not preferable. The pH can be adjusted usingany commonly known pH adjusting agent.

The tin or tin alloy plating film surface treatment aqueous solution ofthe present invention preferably also contains a nitrogen compound. Thenitrogen compound is a compound containing a nitrogen atom in themolecule, and examples include benzotriazole and imidazole, and thelike. Benzotriazole is particularly preferable. The nitrogen compoundcontent as a compound in the surface treatment aqueous solution isbetween 0.1 and 10 g/L, preferably between 0.5 and 5 g/L.

The surface treatment aqueous solution of the present inventiongenerally has a solution temperature between 0° C. and 100° C.,preferably in a range between 10° C. and 70° C., and more preferably therange of room temperature (15° C. to 35° C.).

If necessary, commonly known additives such as surfactants,disinfectants, or solvents, or the like, may also be added to thepresent invention, but these additives are not fundamentally required.The tin or tin alloy plating film surface treatment aqueous solution ofthe present invention can be applied to a tin or tin alloy plating filmsurface by an arbitrary method that can bring the surface treatmentaqueous solution in contact with the plated film surface by a commonlyknown method such as a method that uses an immersion process or a sprayprocess, or the like. Generally, the surface of the tin or tin alloyfilm is washed by water and then the tin or tin alloy plating filmsurface treatment solution of the present invention is applied byimmersing or spraying. The processing time is generally between 5seconds and 120 seconds, preferably between 10 seconds and 60 seconds.Later, a water wash, distilled water rinse, and drying are generallyperformed in order.

The tin or tin alloy plating film surface treatment aqueous solution ofthe present invention is suitable for treating tin or tin alloy platingfilms of components which require electrical connections, for example.Examples of components which require electrical connections include chipcomponents such as chip resistors and chip capacitors, connector pins,bumps, printed circuit boards, lead frames, and other electroniccomponents.

Next, the present invention will be described in detail by presentingembodiments and comparative examples.

The occurrence of whiskers and the plating film thickness of theembodiments and the comparative examples were evaluated as shown below.Furthermore, the appearance after treating with the surface treatmentsolution was observed by the naked eye, and those cases where afavorable plating film was not obtained because the base was exposedthrough the tin plating film, or the like, are shown in the table.

(1) Occurrence of Whiskers

A tin plating film treated with the surface treatment solution wasallowed to sit for three days at room temperature, and after three days,the number of whiskers formed was counted using a metal microscope.Evaluation was made using the following four levels, based on the numberof whiskers that occurred.

O: Very few (between approximately 0 and 3)

1: Few (between approximately 4 and 9)

2: Many (between approximately 10 and 49)

3: Very many (50 or more)

(2) Plating Film Thickness

The tin plating film that had been treated with a surface treatmentaqueous solution was measured using a florescent x-ray measuring device.

EMBODIMENTS 1˜13 AND COMPARATIVE EXAMPLES 1˜14

A patterned two layer CCL material was etched for 30 seconds at roomtemperature, and then washed in order with deionized water, 5% sulfuricacid, and again with deionized water, and then dried. Electroless tinplating was performed for 3 minutes at a bath temperature of 65° C.using a commonly known methanesulfonic acid tin plating bath. Afterwashing with deionized water, the samples were immersed for 60 secondsat room temperature in one of the surface treatment aqueous solutions ofEmbodiments 1 through 8 shown in Table 1 or one of Comparative Examples1 through 14 shown in Table 2. Next, the samples were washed indeionized water, dried, left for three days at room temperature, andthen subjected to the various evaluations. The evaluation results areshown in Table 1 and Table 2.

TABLE 1 Embodiment 1 through 13 Results Surface treatment aqueoussolution Whiskers Film Organic compound containing ConcentrationNitrogen Concentration (4 level thickness Embodiment a carboxyl group(g/L) compound (g/L) PH evaluation) (μm) 1 Malic acid 10 — — 2.27 0 0.512 Maleic acid 10 — — 1.54 0 0.47 3 Maleic acid 25 Benzotriazole 0.5 1.541 0.50 4 Maleic acid 20 Benzotriazole 0.5 1.54 0 0.50 5 Maleic acid 15Benzotriazole 0.5 1.54 0 0.48 6 Maleic acid 10 Benzotriazole 0.5 1.54 00.47 7 Maleic acid 5 — — 1.60 0 0.49 8 Iminodiacetic acid 10 — — 2.29 10.53 9 Citric acid 10 — — 2.2 1 0.50 10 Oxalic acid 5 — — 2.5 1 0.48 11Formic acid 10 — — 2.2 0 0.49 12 Lactic acid 10 — — 2.5 1 0.50 13 Lacticacid 10 Benzotriazole 0.5 2.5 1 0.51

TABLE 2 Comparative Example 1 through 14 Results Surface treatmentaqueous solution Whiskers Film Comparative Organic compound containingConcentration Nitrogen Concentration (4 level thickness Example acarboxyl group (g/L) compound (g/L) PH evaluation) (μm) Comments 1 — — —— 3 0.48 Blank 2 Maleic acid  5 3 3 0.49 3 Maleic acid  5 4 3 0.49 4 — —Benzotriazole 0.5 5 2 0.51 5 Phosphonic acid 10 — 1.26 2 0.49 Copperexposed 6 Aminotrimethylene phosphonic acid 10 — — 1.58 2 0.49 Copperexposed 7 1-hydroxyethylene-1,1-diphosphonic 10 — — 1.70 2 0.47 Copperacid exposed 8 Picolinic acid 10 — — 3.36 3 0.50 9 Ascorbic acid 10 — —2.74 3 0.49 10 (2-benzothiazolyl thio) acetic acid  5 2.5 Not soluble inwater 11 3-(2-benzothiazolyl thio) propionic  5 2.5 Not soluble acid inwater 12 Gluconic acid 20 — 2.6 2 0.52 13 Glutaric acid 10 — — 2.71 20.47 14 Phosphoric acid 10 — — <1 2 0.48 Etching

The surface treatment aqueous solutions of the embodiments prepared withan organic compound containing a carboxyl group and having a pH of 2.5or less were clearly found to have fewer whiskers on the tin platingfilm than samples treated with the surface treatment aqueous solutionsof the comparative examples.

Comparative Examples 2 and 3 were prepared with surface treatmentaqueous solutions where the pH was adjusted to 3 and 4 respectively, byadding an alkali. With these surface treatment aqueous solutions, theeffect of inhibiting whiskers was minimal.

With Comparative Examples 5 through 7, a phosphonic acid-based organiccompound was used. These compounds provided a certain degree ofwhisker-inhibiting effect, but the whisker-inhibiting effect was lowerthan that of the present invention which used an organic compoundcontaining a carboxyl group, and the corrosion was severe so the copperbase was exposed. Note, the aminotrimethylene phosphonic acid ofComparative Example 6 is a compound shown in JP2007-197791.

The organic compounds of Comparative Examples 10 and 11 had lowsolubility in water, were cloudy, and did not form a uniform aqueoussolution, and therefore were not suitable for use as a surface treatmentsolution. Note, Comparative Examples 10 and 11 used a compound shown inJP2004-300466.

Comparative Example 14 used phosphoric acid which is a compound shown inJP2006-307343. Phosphoric acid provided a certain degree ofwhisker-inhibiting effect, but the effect was lower than the organiccompound containing a carboxyl group of the present invention.

By using the tin or tin alloy plating film surface treatment solution ofthe present invention, the occurrence of whiskers on a tin or tin alloyplating film can be effectively prevented by a simple method.

What is claimed is:
 1. A tin or tin alloy plating film surface treatmentaqueous solution consisting of an organic compound chosen from citricacid, malic acid, maleic acid, oxalic acid, glutaric acid, tartaricacid, lactic acid, gluconic acid, picolinic acid, iminodiacetic acid orsalts thereof and a nitrogen containing compound chosen frombenzotriazole or imidazole, water and optionally one or more additivesselected from the group consisting of surfactants, solvents anddisinfectants, the pH of the surface treatment aqueous solution is 2.5or lower.
 2. The tin or tin alloy plating film surface treatment aqueoussolution of claim 1, wherein the concentration of the nitrogencontaining compound is from 0.1-10 g/L.
 3. The tin or tin alloy platingfilm surface treatment aqueous solution of claim 1, wherein the organiccompound is chosen from malic acid, maleic acid or salts thereof.
 4. Thetin or tin alloy plating film surface treatment aqueous solution ofclaim 1, wherein the organic compound ranges between 0.1 and 50 g/L. 5.The tin or tin alloy plating film surface treatment aqueous solution ofclaim 4, wherein the organic compound ranges between 5 and 30 g/L. 6.The tin or tin alloy plating film surface treatment aqueous solution ofclaim 5, wherein the organic compound ranges between 5 and 20 g/L.